PREDICTION OF PORE FLUID PRESSURES IN SEDIMENTARY BASINS

A procedure has been devised for predicting the fluid pressures in a sedimentary basin from the distribution of different rock types, and their burial rates. We have calibrated the procedure with case histories from the North Sea, Gulf of Mexico, Norway and Nile Delta. We show how the procedure is useful for making estimates of the fluid pressures likely to be encountered when drilling a well; the directions of petroleum migration; and the ability of structures to retain oil or gas. The procedure is based on a simple equation which accounts only for the vertical flow of fluids. To include lateral flow, laterally-continuous high permeability layers, such as oil and gas reservoir rocks and aquifers, must be divided into pressure cells. A pressure cell is a part of the layer within which fluids can flow and equalize excess pressures (or overpressures). The fluids of all rocks within the cell are maintained at a nearly constant overpressure (actual pressure minus hydrostatic pressure). Pressure cells which crop out at, or come close to, the surface tend to have very low overpressures, because any excess pressures are dissipated by flow of fluids through the cell to the surface. Deeper pressure cells without a high permeability connection either to the surface or to the shallowest 2 km of sediments tend to be overpressured. The shallower parts of these overpressured cells have higher overpressures than the overlying low permeability rocks. This situation can prevent an accumulation of oil or gas forming. The deeper parts of the same cells have lower overpressures than the overlying permeability rocks, and hence are more likely to contain accumulations of oil and gas. © 1990.